What Is the Crystal Field Theory?

Crystal field theory describes the electrical activity between the atoms of a transitional metal compound. With a focus on the electrical activity between atoms in these compounds, this theory serves to explain the energetic properties of a transitional metal compound, including its color, structure and magnetic field. Though the atoms within these compounds are bonded to one another, the crystal field theory cannot be used to describe these bonds. Incomplete on its own, this theory was combined with ligand field theory in order to incorporate an understanding of the the bonding between atoms.

In the 1930s, crystal field theory was developed by the physicists John Hasbrouck van Vleck and Hans Bleke. These scientists developed their theory alongside, though separate from, ligand field theory. Soon after the development of these two theories, other scientists combined the principles of the two, which are now both studied under modern ligand field theory. The combination of these two theories created a system of equations that was better able to describe the energy fields and molecular bonds within certain types of compounds.

Transition metal compounds can be partially described using crystal field theory. These compounds are made up of atoms of a particular metal which are surrounded by non-metal atoms, called ligands in this context. The electrons of these different atoms interact in ways that can be described using crystal field theory. The bonds that arise from these electron interactions are also described using ligand field theory.

The term crystal field, in crystal field theory, comes from the electrical field generated by a group of ligands. These atoms generate a stable field of energy that a transition metal becomes trapped within. These fields may come in a variety of different geometric shapes. Many transition metal compounds have fields that are in the shape of cubes because such fields are particularly stable and can resist the influence of atoms that are not in the system so that the transitional metal compound remains more stable.

One thing that crystal field theory is particularly good at describing is the coloration of a transitional metal compound. As a relatively stable structure, the electrons in a particular type of compound move towards or away from their nuclei within a limited range. This range determines the color of the substance because it absorbs certain wavelengths of light that correspond to the distance that the electron moves when excited. The wavelengths that are absorbed are not visible in this compound. Instead, the opposite color, as seen on the color wheel, is reflected back, giving the substance its visible color.